Systems and methods for oxidation of synthesis gas tar

ABSTRACT

A process of gasification and the production of synthesis gas. A process of biomass gasification and the reduction or elimination of tars from the hydrocarbon-rich product gas derived from biomass gasification. Systems and methods for the reduction of tar from a synthesis gas derived from biomass gasification are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application which claims the benefitunder 35 U.S.C. §121 of U.S. patent application Ser. No. 12/248,333,filed Oct. 9, 2008, which claims the benefit under 35 U.S.C. §119(e) ofU.S. Provisional Patent Application No. 60/978,593, filed Oct. 9, 2007,the disclosures of each of which are hereby incorporated herein byreference it their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND

1. Technical Field of the Invention

The various embodiments of the present invention relate generally to theprocess of gasification and, the production of synthesis gas. Moreparticularly, the various embodiments of the present invention relate tothe process of biomass gasification and the oxidation of tar from thehydrocarbon-rich product gas derived from biomass gasification.

2. Background of the Invention

Gasification is a process by which either a solid or liquid carbonaceousmaterial (e.g., biomass, coal, petroleum), containing mostly chemicallybound carbon, hydrogen, and oxygen, and a variety of inorganic andorganic constituents, is reacted with air, oxygen, and/or steam.Sufficient energy is provided to produce a primary gaseous productcomprising mostly of CO, H₂, CO₂, H₂O(g), and light hydrocarbons lacedwith volatile and condensable organic and inorganic compounds (e.g.,tars). When gasified with steam and/or oxygen, biomass will produce aproduct gas, sometimes referred to as “synthesis gas” or “syngas” thatis rich in CO and H₂. Synthesis gas can then be catalytically convertedto produce high-value fuels and chemicals.

Unless the raw product gas is combusted immediately followingproduction, it is generally cooled, filtered, and scrubbed with water ora process-derived liquid to remove condensables and any carry-overparticles. Alternatively, the raw gas can undergo eithermedium-temperature (350° C. to 400° C.) or high-temperature (up togasifier exit temperatures) gas cleaning to provide a fuel gas that canbe used in a variety of energy conversion devices, including internalengines, gas turbines, and fuel cells.

A major barrier to the energy efficient and environmentally benignutilization of biomass by gasification the “clean-up” of the productgas. Unless the gas can be used hot, for example in an adjacent boiler,removal of condensable tars from the product gas is usually necessary.For example, one of the most efficient and cleanest ways to use biomassto generate power to date is to use the product gas in a gas combustionturbine; however, this application requires that essentially allcondensable tars are removed from the product gas. Another promisingapplication of a product gas, derived from biomass gasification is forthe synthesis of a wide variety of liquid fuels, including ethanol andhigher alcohols. Similar to the gas turbine applications, the synthesisof alcohols from a biomass gasification product gas also demands theremoval of tars and, if possible, even non-condensable hydrocarbons fromthe product gas.

Several approaches have been tested for the removal of tar from theproduct gas of biomass gasification, such as catalytic and non-catalyticcracking of tar, dry scrubbing with activated carbon and filters, wetscrubbing, steam reformation of tar, and partial oxidation of biomassdirectly to produce synthesis gas. However, according to a studypublished by the National Renewable Energy Laboratory, currentlyavailable technologies for tar removal do not meet the needs of theindustry in terms of cost, performance, and environmental considerations(T. A. Milne; et al., Biomass Gasification Tars, Their Nature,Formation, and Conversion, NREL/TP 570-25357, November 1998).

Accordingly, there is a need for systems and methods for the reductionin the level of tar or destruction of tar from synthesis gas. It is tothe provision of such systems and methods for the reduction in the levelof tar or destruction of tar from synthesis gas that the variousembodiments of the present invention are primarily directed.

SUMMARY

Various embodiments of the present invention relate generally to theprocess of gasification and the production of synthesis gas. Moreparticularly, the various embodiments of the present disclosure relate,to the process, of biomass gasification and the reduction or eliminationof tars from the hydrocarbon-rich product gas derived from biomass,gasification. Briefly described, the present invention relates, tomethods and systems for removing tars from hydrocarbon-rich gases (e.g.,without limitation, product gases from biomass gasification) as well asproducing synthesis gases with improved properties.

An aspect of the present invention comprises a biomass gasificationsystem comprising: a combustor for heating a fluidized particulatematerial; a gasifier to heat a biomass feedstock with the heatedfluidized particulate material to produce a product gas comprising atar; and a reactor to react an oxygen-containing gas with the productgas comprising a tar to at least partially oxidize the tar to producedan oxidized product gas. In an embodiment of the present invention, atleast a portion of the biomass feedstock is converted to char ingasifier and the char is transferred out of the gasifier. In anotherembodiment of the present invention, at least portion of the char istransferred to the combustor and combusted to heat the fluidizedparticulate material. An aspect of the biomass gasification systemcomprises a rate of heat transfer between the heated fluidizedparticulate material and the biomass feedstock sufficient to convert atleast about 70% of the carbon in the biomass feedstock into the productgas at a temperature of about 1200° F. to about 1500° F. In anembodiment of the present invention, the product gas is cooled prior tointroduction into the reactor.

In embodiments of the present invention, the oxygen-containing gas issubstantially pure oxygen. In some embodiments of the present invention,the reactor utilizes a fluidized medium to optimize the oxidation of thetar, such as one or more of calcined dolomite, limestone, olivine sand,and a calcium-containing material. In an exemplary embodiment of thepresent invention, the oxidized product gas has a H₂/CO ratio that is atleast twice as large as the H₂/CO ratio of the product gas comprising atar.

An aspect of the present invention comprises a method for removing tarfrom a gas comprising: introducing a first gas into a reactor, the firstgas comprising tar; introducing a second gas into the reactor, thesecond gas; comprising oxygen; reacting the first gas with the secondgas for time period sufficient to oxidize at least a portion of the tarof the first gas; and producing an at least partially oxidized productgas that has less tar than the first gas comprising a tar. In anembodiment of the present invention, the first gas is a synthesis gas,and the second gas is substantially pure oxygen. In an exemplaryembodiment of the present invention, the oxidized product gas has aH₂/CO ratio that is at least twice as large as the H₂/CO ratio of theproduct gas comprising a tar. In an embodiment of the present invention,the method for removing a tar from a gas can further comprise providinga fluidized medium to the reactor.

An aspect of the present invention comprises, a biomass gasificationmethod, comprising: heating a fluidized particulate material in acombustor; transferring the heated fluidized particulate material to agasifier; introducing a biomass feedstock to the gasifier, wherein heatfrom the fluidized particulate material causes the gasification of atleast a portion of the biomass feedstock to form a tar-containingproduct gas; and introducing the tar-containing product gas and anoxygen-containing gas into a reactor, wherein the oxygen-containing gasreacts with the tar to oxidize at least a portion of the tar of thetar-containing gas to produce an at least partially oxidized product gascomprising less tar than the tar-containing product gas. In oneembodiment of the present invention, at least a portion of the biomassfeedstock is converted to char in the gasifier, and the char istransferred out of the gasifier. In an embodiment of the presentinvention, the oxygen-containing gas is substantially pure oxygen. In anexemplary embodiment of the present invention, the oxidized product gashas a H₂/CO ratio that is at least twice as large as the H₂/CO ratio ofthe product gas comprising a tar.

In an embodiment of the present invention, the biomass gasificationmethod can further comprise introducing a fluidized medium into thereactor. In another embodiment of the present invention, the biomassgasification method can further comprise cooling the tar-containingproduct gas before introducing the product gas into the reactor

Other aspects and features of embodiments of the present invention willbecome apparent to those of ordinary skill in the art, upon reviewingthe following description of specific, exemplary embodiments of thepresent invention. It should be understood, however, that the detaileddescription and the specific examples, while indicating the exemplaryembodiments of the present invention, are, given by way of illustrationonly, since various changes and modifications within the spirit andscope of the present invention will become apparent to those skilled inthe art from this detailed description. These and other objects,features and advantages of the present invention will become moreapparent upon reading the following specification.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic of a biomass gasification system.

DETAILED DESCRIPTION

In an attempt to provide a superior biomass gasification process, thevarious embodiments of the present invention permit the conversion of arange of solid biomass fuels into a medium calorific value gas that canbe directly substituted, for natural gas or as an input for chemicalsynthesis applications. For gas turbine power applications, the use ofbiomass fuels derived from the systems and methods of the presentinvention provide a means to achieve high overall power generationefficiencies without introducing additional greenhouse gases to theenvironment. By converting the biomass into this high energy densitygaseous fuel, significantly higher power generation efficiencies areachieved relative to direct combustion based systems (approximately 40%power generation efficiency compared to a maximum of 25% withconventional biomass systems).

As used herein, the term “biomass” refers to many carbonaceousbiological materials, including but not limited to, plant matter,shredded bark, wood chips, sawdust, sludges, peat, agricultural wastesand residues, animal matter, biodegradable wastes, or combinationsthereof. Cellulosic-type feed materials, which include agriculturalresidues, dewatered sewage sludge, municipal solid waste (predominantlypaper), and fuels derived from municipal solid wastes by shredding andvarious classification techniques can be used in the systems and methodsof the present invention. Also, peat is an acceptable feedstock becauseof its high reactivity, as are lignitic coals.

Unlike other biomass gasification processes, the biomass gasificationsystems and methods of the present invention are not based on starvedair combustion, but rather are based on rapidly heating raw biomass inan air-free environment to generate gas and a solid residue char that isused as a heat source for the biomass heating. Significantly feweremissions are produced in the process because the absence of oxygen inthe gasifier makes it impossible to form dioxins if a chlorinecontaining feed, such as processed municipal solid waster or recycledpaper pulp sludges, is used. In addition, cleaning the high-energydensity, medium-heating value gaseous product is simplified because thegasifier product gas is much lower in volume than the gas from an “airblown” gasifier.

The systems and methods of the present invention are designed to takeadvantage of the unique properties of biomass, such as high reactivity,low ash, low sulfur, and high volatile matter. The reactivity of biomassis such that throughputs in excess of 14,600 kg/hr-m² (3000 lb/hr-ft²)are achieved. In other gasification systems, the throughput is generallylimited to less than 500 kg/hr-m² (100 lb/hr-ft²).

In an exemplary embodiment of the present invention, biomass isindirectly heated using a hot sand stream to produce a medium calorificvalue gas (approximately 17 to 19 kJ/Nm³). The process uses twocirculating fluidized bed reactors as the primary process vessels. Onecirculating fluidized bed is the gasifier in which the biomass is heatedand pyrolyzed produce a product gas that conveys the sand and residualchar from gasification out of the gasifier. After separation of the sandand char from the product gas, the sand and char flow into thecirculating fluidized bed process combustor where the char is combustedto reheat the sand for return to the combustor.

Referring now to the figures wherein like reference numerals representlike parts throughout the several views, exemplary embodiments of thepresent invention will be described in detail. Throughout thisdescription, various components may be identified having specific valuesor parameters, however these items are provided as exemplaryembodiments. Indeed the exemplary embodiments do not limit the variousaspects and concepts of the present invention as many comparableparameters, sizes, ranges, and/or values may be implemented.

An aspect of the present invention comprises a biomass gasificationsystem comprising: a combustor for heating a fluidized particulatematerial; a gasifier to receive a biomass feedstock and the heatedfluidized particulate material, wherein the heated fluidized materialheats the biomass feedstock to produce a product gas comprising a tar;and a reactor to receive an oxygen-containing gas and the product gascomprising a tar, wherein the oxygen-containing gas reacts with theproduct gas to at least partially oxidize the tar.

As used herein, the term “tar” includes, without limitation, a gaseoustar, a liquid tar, a solid tar, a tar-forming substance, or mixturesthereof, which generally comprise hydrocarbons and derivatives thereof.Embodiments of the present invention generally relate to systems andmethods for reducing or eliminating tars from hydrocarbon-rich gases(e.g., product gases from biomass gasification processes), and forsynthesis gas production. These simple, cost-effective, andenvironmentally friendly methods and systems offer a variety ofdesirable features including, without limitation, lower oxygenconsumption, improved recovery of heat, improved H₂/CO ratio,elimination or simplification of down-stream water treatment processes,and/or improved scale-up ability

FIG. 1 illustrates a system 100 comprising a biomass gasificationsystem. As shown in FIG. 1, a gasifier 10 is provided, in which thebiomass B is gasified to produce a product gas 20 that can besubstitutable for natural gas. A combustor 30 is provided, in which thechar remaining after gasification is burned to provide the heat forgasification.

Heat is transferred between the two vessels 10 and 30 via a stream 40 offluidized particulate material (e.g., sand S) that circulates betweenthe gasifier 10 and the combustor 30. The biomass B can be fed into thebase 12 of the gasifier 10, where it mixes with the hot sand S at thebase of the gasifier 10. The sand S at the base of the gasifier 10 isfluidized by the injection of a stream 50 of sufficient steam or othergas.

In an embodiment of the present invention, they rate of heat transferbetween the heated fluidized particulate material S and the biomassfeedstock B is sufficient to convert at least about 70% of the carbon inthe biomass feedstock B into the product gas 20 at a temperature lowerthan about 1300° F. In another embodiment of the present invention, therate of heat transfer between the heated fluidized particulate materialS and the biomass feedstock B is sufficient to convert at least about70% of carbon in the biomass feedstock B into the product gas 20 at atemperature lower that about 1400° F. In yet another embodiment of thepresent invention, the rate of heat transfer between the heatedfluidized particulate material S and the biomass feedstock B issufficient to covert at least about 70% of the carbon in the biomassfeedstock B into the product gas 20 at a temperature lower than about1500° F. In an embodiment of the present invention, the rate of heattransfer between the heated fluidized particulate material S and thebiomass feedstock B is sufficient to convert at least about 70% of thecarbon in the biomass feedstock B into the product gas 20 at atemperature of about 1200° F. to about 1500° F. In another embodiment ofthe present invention the rate of heat transfer between the heatedfluidized particulate material S and the biomass feedstock B issufficient to convert at least about 70% of the carbon in the biomassfeedstock B into the product gas 20 at a temperature of about 1250° F.to about 1450° F. In yet another embodiment of the present invention,the rate of heat transfer between the heated fluidized particulatematerial S and the biomass feedstock B is sufficient to convert at leastabout 70% of the carbon in the biomass feedstock B into the product gas20 at a temperature of about 1250° F. to about 1350° F.

In an exemplary embodiment of the present invention the inside diameterof the gasifier 10 is at least about 36 inches. In an exemplaryembodiment of the present invention, the height of the gasifier 10, isat least about 40 feet.

The fluidized bed provides for very rapid heat transfer between theambient temperature biomass B and the hot sand S. The biomass B gasifiesin this zone, and the product gas 20 generated entrains both thegasifying biomass B and the sand heat carrier out of the gasifier 10.The char/sand mixture 14 can be separated from the product gas 20 bymeans of a cyclone separator 60. The char/sand mixture 14 then flowsfrom a cyclone 62 down into the base of the combustor 30, where the charis burned to reheat the sand S.

The combustor 30 can be a so called “fast fluid bed,” which operatesentrained. The char is burned and the sand/ash mixture is separated froma combustion gas above the combustor 30 by cyclone separators. Theheated sand being much coarser and denser than the ash, is selectivelyremoved in a first stage of separation. The hot sand S separated fromthe flue gas 32 is then returned to the base of the gasifier 10 tocomplete the cycle. Burning the residual biomass char in a separatevessel prevents dilution of the product gas 20 with combustion gases,and thereby allows it to have a higher heating value as well as one thatis constant regardless of the moisture content of the wood.

A reactor 70 receives the tar-containing product gas 20 and anoxygen-containing gas O, wherein the oxygen-containing gas O reacts withthe tan-containing product gas 20 to at least partially oxidize the tar.The oxygen-containing gas O can be many gases with sufficient oxygencontent effective for the sufficient oxidation of tar. For example, theoxygen-containing gas O can be substantially pure oxygen (e.g.,commercially pure grade oxygen), or air, among others. Theoxygen-containing gas O can comprise other components, such as nitrogen,gaseous water, or combinations thereof; among others, as long as theseother components do not substantially interfere with the oxidationreactions. The oxidation of the product gas 20 yields a product gashaving a reduced amount of tar 80 as compared to the input product gas20.

The reactor 70 can further comprise a variety of media M known in theart as a solid circulating or fluidized phase of the circulatingfluidized bed. In various embodiments of the present invention, thesolid circulating or fluidized phase media M may be selected to optimizeperformance of the oxidation reactor (e.g., improving the efficiency ofthe reaction and/or reducing the level of contaminants (e.g., sulfur) inthe gases). For example various solids, including but not limited to,calcined dolomite, limestone, olivine sand, or combinations thereof, areknown to have high tar cracking activities and may be used as the solidfluidized phase in the systems and methods of the present invention. Inanother example calcium-containing materials may be used to facilitatethe scavenging of sulfur in the gases.

The reactor of the present invention can be used in many gasificationsystems, including but not limited to, the biomass gasification systemsdisclosed in U.S. Pat. No. 6,680,137 or U.S. Patent Publication No.2008/0022592, which are hereby incorporated by reference in theirentirety.

An aspect of the present invention comprises a method for reducing theamount of tar in a hydrocarbon-rich gas. In an embodiment of the presentinvention the method comprises introducing a tar-containinghydrocarbon-rich gas into a reactor, introducing an oxygen-containinggas into the reactor, and allowing the hydrocarbon-rich gas andoxygen-containing gas to mix for a sufficient period of time to reducethe amount of tar in the hydrocarbon-rich gas. In an embodiment of thepresent invention, the method comprises reducing the amounts of tarhydrocarbon-rich gas via oxidation. In an embodiment of the presentinvention, an oxygen-containing gas can partially oxidize the tar in ahydrocarbon-rich gas. In another embodiment of the present invention, anoxygen-containing gas can remove or destroy the tar in ahydrocarbon-rich gas. The method further comprises producing an oxidizedproduct gas comprising less tar than tar-containing hydrocarbon-richgas. In an embodiment of the present invention, the systems and methodsof the present invention comprises removing or destroying substantiallyall of the tar in a hydrocarbon-rich gas. As used herein, the term“substantially all” refers to about 99.9900% of the tar in ahydrocarbon-rich gas. One of skill in the art would realize that theextent of tar removal from a hydrocarbon-rich gas would depend on atleast the oxygen content of the oxygen-containing gas, the amount of tarin the hydrocarbon-rich gas, the temperatures of the gases, and/or theextent of mixing the gases among others.

In various embodiments of the present invention, to maximize theeffectiveness and efficiency of the process, the oxygen-containing gag(e.g., substantially pure oxygen) may be rapidly and/or thoroughly mixedin the reactor with the tar-containing hydrocarbon-rich gas. Given thehigh reactivity of the oxygen with the tar-containing hydrocarbon-richgas oxygen may be rapidly consumed. Therefore rapid and thorough mixingof the oxygen-containing gas and the tar-containing hydrocarbon-rich gasallow a portion, a majority or more of the tar to access and react withthe oxygen before the oxygen is consumed.

Reactors suitable for purposes of the present invention are well-knownin the art. In various embodiments, methods of the present invention mayutilize reactor systems that can create rapid mixing of thetar-containing hydrocarbon-rich gas with the oxygen-containing gas.These reaction systems can comprise various types of fluidized-beds,including, without limitation, conventional bubbling bed (“CBB”)fluidized reactors, circulating fluid-bed (“CFB”) reactors, andmulti-solid fluid-bed (“MSFB”) reactors, which enable rapid mixing ofreactants and have high heat and mass transfer rates. In an embodimentof the present invention, the reactor can comprise a partial oxidationreactor.

In an embodiment of the present invention, the reactor can be a CBBreactor, which generally operates at gas velocities that are relativelysmall multiples of the minimum fluidization velocity, such as, withoutlimitation, in the range of about 2 to about 5 ft/sec. CBB reactors havelong been used in the chemical process industry, as well as incombustion applications, where the fluidized particles may be chemicallyactive. For example limestone and dolomite are widely used as thefluidized phase for CBB reactors to capture sulfur.

In another embodiment of the present invention the reactor can be a CFBreactor, which generally operates at much higher gas velocities than aCBB reactor (e.g., without limitation, about 10 ft/sec to about 30ft/sec or more), and is therefore more compact than a conventional fluidbed reactor. By operating at very high solids circulation rates, thistype of reactor may be able to maintain high solids densities in the beddespite operating at velocities much higher than the elutriationvelocities of the solids. In addition, the high solid circulation ratesmay eliminate bubble formation, often associated with CBB reactors,where bubbles form in the bed and rise up through the so-called emulsionphase. When the mass transfer rate between the bubbles and emulsionphase is lower than the oxidation rate, it is possible that some of thetar may escape oxidation.

In yet another embodiment, the reactor can be a MSFB reactor, such as,without limitation, the MSFB reactor disclosed in U.S. Pat. No.4,154,581, which is herein incorporated by reference in its entirety. Ascompared to CFB reactors, MSFB reactors generally use a bed of muchcoarser particles (e.g., without limitation ¼ inch dense ceramicspheres) that may create a higher solids density and a highly mixedturbulent zone at the base of the partial oxidization reactor. The rapidand thorough mixing created by the presence of the dense phase particlescreates conditions suitable for the complete oxidation of tars in thetar-containing hydrocarbon-rich gas. In addition, because of the highgas velocities used the MSFB may also be a compact reactor.

In various embodiments of the present invention, a solids circulationcircuit of the reactor can comprise a cyclone for removing solids and adowncomer for returning and recycling the solids to the fluid bed.Solids rates may be controlled, such as, without limitation, bycontrolling the solids inventory in the system as well as by usingmechanical and non-mechanical valves known in the art.

An aspect of the present invention comprises a method for removing tarfrom a gas, comprising: introducing a first gas into a reactor, thefirst gas comprising a tar; introducing a second gas into the reactor,the second gas comprising oxygen; and reacting the first gas with thesecond gas for time period sufficient to oxidize at least a portion ofthe tar of the first gas. In an embodiment of the present invention, thefirst gas can comprise many gases comprising tar, including but notlimited to, a hydrocarbon-rich gas, such as synthesis gas.

The sufficient mixture of oxygen with the tar-containinghydrocarbon-rich gases may play a role in the minimization of thepresence of tar in the product gas before the oxygen provided to thereactor is consumed. Under some circumstances, for example, the reactionrate of the oxygen with tar-containing hydrocarbon-rich gas can becontrolled by regulating the configuration and performance of thereactor to allow the mixing rates to be equal to or higher than thereaction rate of the tar-containing hydrocarbon-rich gas with oxygen. Inone embodiment, a plurality of injection ports for the oxygen-containinggas can be employed in the reactor to increase mixing rates of oxygenwith tar-containing gas. As used herein, the term “plurality” means morethan one. The number and configuration of these ports may depend on anumber of factors, such as, the type of fluid bed employed and its size,which can be determined by a person of ordinary skill in the art withoutthe need for undue experimentation. For example, cold model testing maybe used to establish the optimum configuration of these ports as well asinjection velocities and other operational parameters.

In an embodiment of the present invention the tar-containinghydrocarbon-rich gas can be heated or cooled prior to injection into thereactor (e.g., cooling a tar-containing hydrocarbon-rich gas to apredetermined temperature). In one embodiment of the present invention,the tar-containing hydrocarbon-rich gas can be a pre-heated gas, such asthe product gas from biomass gasification. In an exemplary embodiment ofthe present invention the temperature of the tar-containinghydrocarbon-rich gas can be cooled to a level where the rate ofoxidation may be reduced to a point where complete or substantiallycomplete mixing of the oxygen-containing gas and the tar-containinghydrocarbon-rich gas may be achieved before the oxygen is consumed. Inthe examples shown in Tables 1, 2 and 3, hot product gases with atemperature about 1200° F. to about 1300° F. were mixed with oxygenhaving a temperature of about 60° F. Temperature increases fromoxidation were on the order of about 300° F. to about 600° F. dependingon the amount of oxygen injected into the reactor. The product gas canbe cooled from about 1200° F. to about 800° F. without any tarcondensation, which indicates that there is considerable flexibility inusing gas cabling to control oxidation rates.

In another embodiment of the present invention, the oxygen-containinggas can be diluted with steam or another gas to reduce the rate ofoxidation to a level where complete or substantially complete mixing ofoxygen-containing gas and tar-containing hydrocarbon-rich gas may beachieved before the oxygen is consumed.

In an embodiment of the present invention, the tar-containinghydrocarbon-rich gas can be pre-heated or pre-cooled to a predeterminedtemperature suitable for tar reduction or destruction before introducingthe gas into the reactor. In another embodiment of the presentinvention, the tar-containing hydrocarbon-rich gas, such as the productgas from a biomass gasification process, can be pre-cleaned priorproviding the gas to the reactor to separate the char and/or sand fromthe gasifier product gas.

In addition, embodiments of the present invention provide a method forproducing synthesis gas, which comprises introducing a tar-containinghydrocarbon-rich gas into a reactor, introducing an oxygen-containinggas into the reactor, and allowing the hydrocarbon-rich gas and theoxygen-containing gas to mix for a sufficient period of time to reducethe amount of tar in the tar-containing hydrocarbon gas. The synthesisgas produced therewith may comprise a variety of desirable featuresincluding without limitation, improved H₂/CO ratio, reduced or minimizedcontamination (e.g., tars, hydrogen sulfide, or other sulfur-basedcontaminants), and/or improved total heating value (although the measureof Btu/scf decreases as the hydrocarbons are eliminated, the totalheating value increases because the total gas volume increases followingoxidation).

Although the exemplary embodiments of the present invention are directedtowards systems and methods of biomass gasification, one of ordinaryskill in the art would realize that the systems and methods of thepresent invention are applicable to many gasification processes for theconversion of many carbonaceous materials, including but not limited tothe gasification of coal, petroleum, natural gas, alcohols, and otherhydrocarbon-containing materials.

As used herein and in the appended claims the singular forms “a,” “an,”and “the” include plural references, unless the content clearly dictatesotherwise. Thus for example, reference to “a hydrocarbon-rich gas”includes a plurality of such hydrocarbon-rich gases and equivalentsthereof known to those skilled in the art, and reference to “the gas” isa reference to one or more such gases and equivalents thereof known tothose skilled in the art, and so forth. All publications, patentapplications, patents, and other references mentioned herein areincorporated by reference in their entirety.

It should be understood, of course, that the foregoing relates only toexemplary embodiments of the present invention and that numerousmodifications or alterations may be made therein without departing fromthe spirit and the scope of the invention as set forth in thisdisclosure.

Although the exemplary embodiments of the present invention are providedherein, the present invention is not limited to these embodiments. Thereare numerous modifications or alterations that may suggest themselves tothose skilled in the art.

The present invention is further illustrated by way of the examplescontained herein, which are provided for clarity of understanding. Theexemplary embodiments should not to be construed in any way as imposinglimitations upon the scope thereof. On the contrary, it is to be clearlyunderstood that resort may be had to various other embodiments,modifications, and equivalents thereof which, after reading thedescription herein, may suggest themselves to those skilled in the artwithout departing from the spirit of the present invention and/or thescope of the appended claims.

Therefore, while embodiments of this invention have been described indetail with particular reference to exemplary embodiments those skilledin the art will understand that variations and modifications can beeffected within the scope of the invention as defined in the appendedclaims. Accordingly, the scope of the various embodiments of the presentinvention should not be limited to the above discussed embodiments andshould only be defined by the following claims and all equivalents.

EXAMPLES Example 1 Partial Oxidation of a Product Gas

A number of simulations have been performed based on equilibriumcalculations to examine the ability of the systems and methods of thepresent invention to achieve the high levels of tar destruction requiredfor many applications. The results are summarized in Tables 1-3. Becauseof the very high rates of reaction and the high temperatures involved,it is expected that chemical equilibrium will be achieved. Another issuefor successful tar destruction, the rapid and thorough mixing ofinjected O₂ with the product gas, will be achieved by the meanssummarized, supra including a more fluid dynamic modeling. Establishinga desirable fluid dynamic modeling for a particular facility may beachieved using techniques known in the art.

Table 1 shows the results of a representative equilibrium calculationusing the tar constituents in a typical SilvaGas product gas, which isproduced in the SilvaGas Gasification Plant in Burlington Vt., with bothgasification and combustion being carried out in circulating fluidizedbeds operating at very solids fluxes (30,000 to 60,000 lbs/sq-ft/sec).The gas to the partial oxidation unit enters at a temperature of about1207° F. As shown in Table 1, constituents from benzene on down the listare referred to as tars. Gas exits the partial oxidizer at about 1827°F. having destroyed a substantial amount of tar.

TABLE 1 Partial Oxidation of the Product Gas from a Nominal 800 TPDSilvaGas Gasification Plant Properties Overall Vapor Liquid Solid Topartial oxidation unit Temperature, ° F. 1206.792 Pressure, psia 14.696Std sp. gr. * air = 1 0.759 Molar flow, lbmol/h 4138.620 4138.620 0.0000.000 Mass flow, lb/h 90928.703 90928.703 0.000 0.000 Avg. mol. wt.21.971 21.971 0.000 0.000 Actual density, lb/ft³ 0.018 0.018 0.000 0.000Flowrates in lbmol/h Oxygen 358.4590 358.4590 0.0000 0.0000 Hydrogen458.2348 458.2348 0.0000 0.0000 Carbon Dioxide 244.1404 244.1404 0.00000.0000 Carbon Monoxide 1014.1410 1014.1410 0.0000 0.0000 Methane345.5455 345.5455 0.0000 0.0000 Ethylene 116.4351 116.4351 0.0000 0.0000Ethane 15.0238 15.0238 0.0000 0.0000 Nitrogen 15.0238 15.0238 0.00000.0000 Water 1555.0109 1555.0109 0.0000 0.0000 Hydrogen Sulfide 2.05592.0559 0.0000 0.0000 Ammonia 0.0185 0.0185 0.0000 0.0000 Benzene 0.59330.5933 0.0000 0.0000 Toluene 0.6718 0.6718 0.0000 0.0000 Ethylbenzene0.0785 0.0785 0.0000 0.0000 O-Xylene 0.0562 0.0562 0.0000 0.0000M-Xylene 0.0562 0.0562 0.0000 0.0000 P-Xylene 0.0562 0.0562 0.00000.0000 Styrene 1.0382 1.0382 0.0000 0.0000 1,3,5-Trimethylb 0.01160.0116 0.0000 0.0000 Phenol 5.3220 5.3220 0.0000 0.00001,2,4-Trithethylb 0.0349 0.0349 0.0000 0.0000 O-Cresol 0.8347 0.83470.0000 0.0000 M-Cresol 0.6805 0.6805 0.0000 0.0000 P-Cresol 0.68050.6805 0.0000 0.0000 Naphthalene 2.9082 2.9082 0.0000 0.0000Acenaphthene 0.6514 0.6514 0.0000 0.0000 Fluorene 0.1777 0.1777 0.00000.0000 Phenanthrene 0.3054 0.3054 0.0000 0.0000 Anthracene 0.1353 0.13530.0000 0.0000 Fluoranthene 0.0872 0.0872 0.0000 0.0000 Pyrene 0.11490.1149 0.0000 0.0000 Benzanthracene 0.0349 0.0349 0.0000 0.0000 Chrysene0.0031 0.0031 0.0000 0.0000 Nitric Oxide 0.0000 0.0000 0.0000 0.0000Nitrogen Dioxide 0.0000 0.0000 0.0000 0.0000 Sulfur Dioxide 0.00000.0000 0.0000 0.0000 Exit oxidation unit Temperature, ° F. 1826.5321826.532 0.000 0.000 Pressure, psia 14.696 14.696 0.000 0.000 Std sp.gr. * air = 1 0.623 0.623 0.000 0.000 Molar flow, lbmol/h 5036.0245036.024 0.000 0.000 Mass flow, lb/h 90931.039 90931.039 0.000 0.000Avg. mol. wt. 18.056 18.056 0.000 0.000 Actual density, lb/ft³ 0.0110.011 0.000 0.000 Flowrates in lbmol/h Oxygen 0.000 0.000 0.0000 0.0000Hydrogen 1806.3513 1806.3513 0.0000 0.0000 Carbon Dioxide 569.3259569.3259 0.0000 0.0000 Carbon Monoxide 1413.1492 1413.1492 0.0000 0.0000Methane 0.0337 0.0337 0.0000 0.0000 Ethylene 0.0000 0.0000 0.0000 0.0000Ethane 0.0000 0.0000 0.0000 0.0000 Nitrogen 15.0285 15.0285 0.00000.0000 Water 1230.0728 1230.0728 0.0000 0.0000 Hydrogen Sulfide 2.05482.0548 0.0000 0.0000 Ammonia 0.0085 0.0085 0.0000 0.0000 Benzene 0.00000.0000 0.0000 0.0000 Toluene 0.0000 0.0000 0.0000 0.0000 Ethylbenzene0.0000 0.0000 0.0000 0.0000 O-Xylene 0.0000 0.0000 0.0000 0.0000M-Xylene 0.0000 0.0000 0.0000 0.0000 P-Xylene 0.0000 0.0000 0.00000.0000 Styrene 0.0000 0.0000 0.0000 0.0000 1,3,5-Trimethylb 0.00000.0000 0.0000 0.0000 Phenol 0.0000 0.0000 0.0000 0.0000 1,2,4-Trimethylb0.0000 0.0000 0.0000 0.0000 O-Cresol 0.0000 0.0000 0.0000 0.0000M-Cresol 0.0000 0.0000 0.0000 0.0000 P-Cresol 0.0000 0.0000 0.00000.0000 Naphthalene 0.0000 0.0000 0.0000 0.0000 Acenaphthene 0.00000.0000 0.0000 0.0000 Fluorene 0.0000 0.0000 0.0000 0.0000 Phenanthrene0.0000 0.0000 0.0000 0.0000 Anthracene 0.0000 0.0000 0.0000 0.0000Fluoranthene 0.0000 0.0000 0.0000 0.0000 Pyrene 0.0000 0.0000 0.00000.0000 Benzanthracene 0.0000 0.0000 0.0000 0.0000 Chrysene 0.0000 0.00000.0000 0.0000 Nitric Oxide 0.0000 0.0000 0.0000 0.0000 Nitrogen Dioxide0.0000 0.0000 0.0000 0.0000 Sulfur Dioxide 0.0001 0.0001 0.0000 0.0000

In addition to at least partial destruction of tars and elimination ofhydrocarbon gases (except for a trace of methane), the H₂/CO ratio issignificantly improved—from 0.45 for the input product as to 1.3 for theoutput gas from the partial oxidation reactor (i.e., the tar destructionreactor), which makes it more suitable for the production of synthesisgas. Sulfur in the output gas will remain as H₂S because of the highlyreducing nature of the gas from the partial oxidation reactor andthereby will be able to be captured using limestone or the like as thefluidizing media or in a separate sulfur guard unit. The destruction oftars, including phenols and cresols, will greatly simplify thedown-stream water treatment process which otherwise can be an expensiveprocessing step. Also of great importance is that all these benefits areachieved with no loss of product gas total energy, which, in fact,actually has a small increase due to the endothermic reforming reactionsdriven by the high steam content of the product gas (See also Table 3).

Example 2 Partial Oxidation of a Hydrocarbon-Rich Gas Containing Toluene

Table 2 shows the results of a representative equilibrium calculationusing gas mixtures containing only one tar species. A software programbased on minimizing the Gibbs free energy was used to computeequilibrium compositions of reacting gas mixtures as well as the heatbalance, assuming the system is adiabatic. The software enabledexamination of factors, such as gas compositions, temperatures, oxygenconsumption, and potential loss of energy in the product gas. Programssimilar to the one used by the inventor are widely known in the art, forexample as component in the well known chemical design program ChemCad.

In the particular example, the only tar species in the input reactinggas mixture was toluene, which was used as a tar surrogate. The reactantgas composition was taken directly from the heat and material balancemodel at 25% moisture and default values for the other parameters. Thegas composition was based on a proprietary heat and material balancemodel developed from a pilot plant as well as data from a commercialsized plant. The calculations were based on per pound of bone dry wood.

The software simulation showed that at every level of oxygen tested, theprocess achieved elimination of the tar surrogate well as elimination ofhydrocarbon gases by reforming and partial oxidation. The combustionenergy of the partial oxidation products exceeded that of the gasifierproduct gas until a partial oxidation temperature of 1900° F., at whichpoint they are about equal. While there was no loss of total combustionenergy, the volume of moisture-free product gas was substantially higherafter partial oxidation because of the reforming of the higher energydensity hydrocarbons. For example, the untreated dry product gas has ashigher heating value (“hhv”) of about 470 Btu/scf and the oxidizedproduct gas has an hhv of about 275 Btu/scf.

TABLE 2 Partial Oxidation of Hydrocarbon-Rich Gas Containing TolueneConditions: Adiabatic T and composition at constant P No. moles Molefraction Reactants C₂H₄ 1.9700 × 10−03 0.02791 C₂H₆ 2.6000 × 10−04 3.684× 10−03 CH₄ 5.9000 × 10−03 0.08359 CO 1.7400 × 10−02 0.24653 CO₂ 4.2000× 10−03 0.05951 H₂ 7.8000 × 10−03 0.11051 H₂O 2.6500 × 10−02 0.37546Toluene 3.0000 × 10−04 4.250 × 10−03 O₂ 6.2500 × 10−03 0.08855 ProductsC₂H₄ 2.1024 × 10−13 2.442 × 10−12 C₂H₆ 3.2447 × 10−15 3.769 × 10−14 CH₄2.5411 × 10−07 2.952 × 10−06 CO 2.4637 × 10−02 0.28621 CO₂ 9.4227 ×10−03 0.10946 H₂ 3.0702 × 10−02 0.35667 H₂O 2.1318 × 10−02 0.24765Toluene 2.4153 × 10−37 2.806 × 10−36 O₂ 5.2943 × 10−16 6.150 × 10−15Properties Reactants Products Temperature (K) 970 1315.6 Pressure (atm)1 1 Volume Ratio 1 1.654 Moles Prod/React 1.219602

As further depicted in Table 3, the output gas leaving the partialoxidation unit has a significantly higher temperature than that of theinput gas entering the reactor. The substantial increase in the productgas temperature allows increased recovery of sensible heat with nolimitations due to tar condensation. In addition, embodiments of thepresent invention provide greater compression requirements—from about1.7 to about 1.8 that of the scrubbed product gas. When dry wood is usedas the materials for biomass gasification, oxygen requirements fordestructing tars range from about 0.16 to about 0.2 tons of oxygen perton of dry wood, adding only a limited amount to the cost of production.

TABLE 3 Summary of Partial Oxidation Results Oxygen lb moles/lb dry wood0.005 0.0055 0.00575 0.006 0.00625 tons/ton dry wood 0.16 0.176 0.1840.192 0.2 Partial Oxidation Temperature, ° F. 1593 1719 1782 1846 1909HHV Oxidation Product Gas, Btu/scf 274 275 276 276 277 Ratio of totalcombustion energy 1.05 1.035 1.03 1.02 1.01 (oxidation product/scrubbedgas) Partial Oxidation Gas Composition (vol % dry) CO 34.3 35.9 36.737.3 38 CO₂ 15.5 15 14.8 14.7 14.5 H₂ 50.2 49.1 48.5 48 47.5 Total 100100 100 100 100 mole ratio partial 1.82 1.78 1.77 1.75 1.73 oxgas/scrubbed gas (dry)

While the invention has been disclosed in its preferred forms it will beapparent to those skilled in the art that many modifications additionsand deletions can be made therein without departing from the spirit andscope of the invention and its equivalents as set forth in the followingclaims.

What is claimed is:
 1. A method for removing tar from a gas, the methodcomprising: contacting a first gas comprising tar with a second gascomprising oxygen for time period sufficient to effect oxidation of atleast a portion of the tar in the first gas, thus producing an oxidizedproduct gas that comprises less tar than the first gas; and adjustingthe rate of oxidation to a rate at which substantially complete mixingof the first gas with the second gas is achieved before the oxygen inthe second gas is completely consumed.
 2. The method of claim 1, whereinthe second gas is selected from the group consisting of substantiallypure oxygen, air, and oxygen-enriched air.
 3. The method of claim 1,wherein the first gas comprises synthesis gas.
 4. The method of claim 3,wherein the molar ratio of H₂/CO in the oxidized product gas is greaterthan the molar ratio of H₂/CO in the first gas.
 5. The method of claim3, wherein the molar ratio of H₂/CO in the oxidized product gas is atleast twice as large as the molar ratio of H₂/CO in the first gas. 6.The method of claim 1, wherein contacting the first gas with the secondgas is performed in the presence of a fluidized medium.
 7. The method ofclaim 6, wherein the fluidized medium is selected from the groupconsisting of calcined dolomite, limestone, olivine sand, calciumcontaining materials, and combinations thereof.
 8. The method of claim1, wherein the rate of oxidation is adjusted by cooling the second gasprior to contacting thereof with the first gas.
 9. The method of claim 8further comprising producing at least a portion of the first gas byintroducing a biomass feedstock to a gasifier, wherein heat from afluidized particulate material causes the gasification of at least aportion of the biomass feedstock to form a tar-containing product gas,and wherein the at least a portion of the first gas is cooled to atemperature less than the gasification temperature and greater than thetemperature at which at least a portion of the tar condenses prior tocontacting thereof with the second gas.
 10. The method of claim 1,wherein the rate of oxidation is adjusted by utilizing a second gascomprising a diluent in addition to oxygen.
 11. A method for removingtar from a gas, the method comprising: producing a first gas, comprisingtar by introducing a biomass feedstock to a gasifier, wherein heat froma fluidized particulate material therein causes the gasification of atleast a portion of the biomass feedstock to form a tar-containingproduct gas; and contacting at least a portion of the first gas with asecond gas comprising oxygen for time period sufficient to effectoxidation of at least a portion of the tar in the first gas, thusproducing an oxidized product gas that comprises less tar than the firstgas.
 12. The method of claim 11 further comprising: heating a fluidizedparticulate material in a combustor; and transferring the heatedfluidized particulate material to the gasifier.
 13. The method of claim12, wherein at least a portion of the biomass feedstock is converted tochar in the gasifier and wherein the method further comprisestransferring at least a portion of the char out of the gasifier.
 14. Themethod of claim 13, further comprising (a) separating fluidizedparticulate material and optionally char produced in the gasifier fromthe tar-containing product gas, (b) transferring at least a portion ofthe char to the combustor and combusting the transferred char to heatthe fluidized particulate material, or both (a) and (b).
 15. The methodof claim 11 further comprising cooling the at least a portion of thetar-containing product gas in the first gas prior to contacting thefirst gas with the second gas.
 16. The method of claim 11 furthercomprising operating the gasifier at a temperature in the range of fromabout 1200° F. to about 1500° F.
 17. The method of claim 16, wherein therate of heat transfer between the heated fluidized particulate materialand the biomass feedstock is sufficient to convert at least about 70% ofthe carbon in the biomass feedstock into the tar-containing product gas.18. The method of claim 11, further comprising carrying out thecontacting of the first gas with the second gas in a fluidized bedreactor selected from the group consisting of conventional bubbling bedreactors, circulating fluid bed reactors, multi-solid fluid bedreactors, and combinations thereof.
 19. The method of claim 11, whereinthe oxidized product gas has a total combustion energy that is greaterthan the total combustion energy of the first gas, comprisessubstantially no tar, or both.